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I was installing a tl3141 into a headphone amplifier circuit and it appears to break into ~250mVp-p oscillation at 8-10MHz on the lower half of the waveform when fed a 1kHz sine wave.

Is that even possible given the specs of the opamp (GBP 1.1MHz, Slew rate 1.3V/μSec)? Given the measured oscillation the output would have to swing at somewhere around 2.75V/μSec which is well beyond that spec.

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  • \$\begingroup\$ Depends on the opamp. For example the low bandwidth LM741 was a moderately fast LM748 with a built-in compensation capacitor. If the fast stages have feedback paths (like poorly designed supplies) independent of that cap, it's possible. \$\endgroup\$ – Brian Drummond Sep 28 '17 at 13:51
  • \$\begingroup\$ If my measurements/math are correct the output is swinging 250mV in .09μSec which is a slew rate of ~2.75V/μSec and double the spec. \$\endgroup\$ – Frosty Sep 28 '17 at 13:57
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Some Class AB power amplifiers use a composite PNP/NPN transistor pair to drive current on the low-side swing. LM380, and your TL3141 use this topology. For the LM380, appnotes caution that 5-10 MHz oscillations can be experienced, known to audio insiders as "low-side fuzzies". A Zobel network is suggested as a cure (A series RC from output to ground). Good high-frequency power-supply bypassing can help too.

enter image description here

EDIT: From "Audio HANDBOOK National" (1976),

...The classic class B is merely a PNP and NPN capable of huge currents but since the IC designer lacks good quality PNP's, a number of compromises results. Figure 4.1.4b shows the bottom side PNP replaced with a composite PNP/NPN arrangement. Unfortunately, Q2/Q3 form a feedback loop which is quite inclined to oscillate in the 2-5MHz range. Although the oscillation frequency is well above the audible range, it can be troublesome when placed in proximity to an RF receiver. Among the stabilization techniques that are in use, with varying degrees of success are:

  • Placing an external RC from the output pin to ground to lower the gain of the NPN. This works pretty well and appears on numerous data sheets as an external cure.

  • Utilizing device geometry methods to improve the PNP's frequency response. This has been done successfully in the LM377, LM378, LM379. The only problem with this scheme is that biasing the improved PNP reduces the usable output swing slightly, thereby lowering output power capability.

  • Addition of resistance in series with either the emitter or base of Q3.

  • Making Q3 a controlled gain PNP of unity, which has the added advantage of keeping gain more nearly equal for each half cycle.

  • Adding capacitance to ground from Q3's collector.

These last three work sometimes to some degree at most current levels.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you! That looks like my exact issue. I edited your answer to include a screenshot of the appnote, hope you don't mind. \$\endgroup\$ – Frosty Sep 28 '17 at 14:32
  • \$\begingroup\$ Does "low-side fuzzies" have any synonyms? I'm trying to find more discussions of the phenomenon but can't find much with that term. \$\endgroup\$ – Frosty Sep 28 '17 at 14:37
  • \$\begingroup\$ @Frosty Am trying to find the reference, seen in an ancient ap-note book that dissects IC power-amp internals . Have to dig deeper...if I find it, will re-edit. \$\endgroup\$ – glen_geek Sep 28 '17 at 14:53
  • \$\begingroup\$ ti.com/lit/an/snaa086/snaa086.pdf page three. I stuck a screenshot in your answer, but it is waiting for peer review. Oh, you are talking about the fuzzies... \$\endgroup\$ – Frosty Sep 28 '17 at 15:14
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    \$\begingroup\$ Upvoted! Also even without a CFP output, the output stage of a discrete amp (or an opamp) can oscillate at frequencies way above the amp's GBW if there is too much inductance in the supply or in the gates/bases wiring, this can create positive feedback and oscillations. For an opamp, there is very little inductance inside the circuit, but the supply matters, a catastrophically bad decoupling can make it oscillate. \$\endgroup\$ – peufeu Sep 28 '17 at 16:32
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GBP as stated for the amplifier, if used as such, does not mean any input-to-rail or rail-to-input effects cannot be far higher bandwidth, and create surprises like that if there is anything unsatisfactory about power bypassing or filtering.

Also, with common opamps the GBP is based on a voltage gain. Depending on input capacitance, even at the maximum frequency, most power available into the input is not used. Neither is most of the power available at the output usually consumed. Now all it takes is some intentional or unintentional passive network (tank circuit, piezoelectric part, tapped coil, transformer) providing voltage gain, even at a power loss...

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